In recent years, the exploration of crude oil and natural gas and pipeline construction have been actively performed in very cold regions such as the North Sea, Canada, and Alaska, because of a rise in the price of crude oil, the demand for the diversification of sources of energy allow, and so forth, since the oil crisis. Furthermore, for example, highly corrosive sour gas fields where their developments were once abandoned are actively under way.
For pipelines, high-pressure operation tends to be performed using large-diameter pipes to increase the transport efficiency of natural gas and oil. To withstand high-pressure operation of pipelines, thick-walled steel pipes need to be used as transport pipes. Thus, UOE steel pipes made from thick-walled steel sheets have been increasingly used. Nowadays, however, a strong demand for a further reduction in the cost of pipeline construction, the undersupply of UOE steel pipes, and so forth strongly require a reduction in the material cost of steel pipes. Instead of UOE steel pipes made from thick-walled steel sheets, high strength electric resistance welded steel pipes or high strength spiral steel pipes, which are made from coiled hot rolled steel sheets (hot rolled steel strips) with high productivity and at lower cost, have been increasingly used as transport pipes.
These high strength steel pipes are required to maintain their excellent low-temperature toughness from the viewpoint of preventing the bursting of transport pipes. To produce steel pipes having both high strength and high toughness, for steel sheets serving as materials for steel pipes, attempts have been made to achieve an increase in strength by transformation strengthening using accelerated cooling after hot rolling, precipitation strengthening using precipitates, such as Nb, V, and Ti, of alloy elements, and so forth, and an increase in toughness by forming a finer microstructure using controlled rolling and so forth.
Furthermore, transport pipes used for transporting crude oil and natural gas that contain hydrogen sulfide are required to have excellent sour gas resistance, such as hydrogen induced cracking resistance (HIC resistance) and stress corrosion cracking resistance, in addition to the characteristics, for example, high strength and high toughness.
For such a request, for example, Japanese Unexamined Patent Application Publication No. 08-319538 discloses a method for producing a low yield ratio and high strength hot rolled steel sheet having excellent toughness, the method including the steps of hot-rolling steel that contains, on a mass percent, 0.005% to less than 0.030% C, 0.0002% to 0.0100% B, one or both elements selected from 0.20% or less Ti and 0.25% or less Nb in amounts such that (Ti+Nb/2)/C is 4 or more, and Si, Mn, P, S, Al, and N in appropriate amounts, cooling the steel at a cooling rate of 5 to 20° C./s, coiling the steel at a temperature in the range of higher than 550° C. to 700° C. or lower, whereby the microstructure is composed of ferrite and/or bainitic ferrite, and the amount of solid solution carbon in grains is in the range of 1.0 to 4.0 ppm. The technique described in JP '538 seems to provide a low yield ratio and high strength hot rolled steel sheet having excellent toughness, weldability, and sour gas resistance without causing the nonuniformity of the material in the thickness direction and longitudinal direction. However, in the technique described in JP '538, the amount of solid solution carbon in crystal grains is 1.0 to 4.0 ppm. Hence, heat input during girth welding is disadvantageously liable to cause grain growth. That is, coarse grains are formed in a welded heat affected zone. This is liable to cause a deterioration in the toughness in the welded heat affected zone of a girth welded portion.
Japanese Unexamined Patent Application Publication No. 09-296216 discloses a method for producing a high-strength steel sheet having excellent hydrogen induced cracking resistance, the method including terminating hot rolling of a steel slab at a temperature of Ar3+100° C. or higher, the steel slab containing, on a mass percent, 0.01%-0.12% C, 0.5% or less Si, 0.5%-1.8% Mn, 0.010%-0.030% Ti, 0.01%-0.05% Nb, and 0.0005%-0.0050% Ca to satisfy a carbon equivalent of 0.40 or less and a Ca/O of 1.5 to 2.0; performing air cooling for 1 to 20 seconds; cooling the steel sheet from the Ar3 point or higher to 550° C. to 650° C. in 20 seconds; and coiling the steel sheet at 450° C. to 500° C. The technique described in JP '216 seems to provide a steel sheet for a transport pipe specified by API X60 to X70 grade, the steel sheet having hydrogen induced cracking resistance. However, in the technique described in JP '216, in the case of a steel sheet having a large thickness, a desired cooling time is not ensured. To ensure desired properties, further improvement in cooling capacity is disadvantageously needed.
Japanese Unexamined Patent Application Publication No. 2008-056962 discloses a method for producing a thick high-strength steel plate for a transport pipe having excellent hydrogen induced cracking resistance, the method including heating steel containing, on a mass percent, 0.03%-0.06% C, 0.01%-0.5% Si, 0.8%-1.5% Mn, 0.0015% or less S, 0.08% or less Al, 0.001%-0.005% Ca, and 0.0030% or less O, Ca, S, and O satisfying a specific relationship; performing accelerated cooling at a cooling rate of 5° C./s or more from the Ar3 transformation point to 400° C. to 600° C.; thereafter rapidly reheating the steel plate at a heating rate of 0.5° C./s or more in such a manner that the surface temperature of the steel plate reaches 600° C. or higher and that a temperature at a middle position of the steel plate in the thickness direction reaches 550° C. to 700° C., whereby the difference in temperature between the surface of the steel plate and the middle position of the steel plate in the thickness direction when the reheating is completed is 20° C. or higher. The technique described in JP '962 seems to provide a steel plate in which the fraction of a second phase in the metal microstructure is 3% or less and in which the difference in hardness between a surface layer and the middle position of the steel plate in the thickness direction is 40 points or less in terms of Vickers hardness, the thick steel plate having excellent hydrogen induced cracking resistance. However, in the technique described in JP '962, disadvantageously, the reheating step is needed, making the production process complex. Furthermore, it is necessary to install a reheating apparatus and so forth.
Japanese Unexamined Patent Application Publication No. 2001-240936 discloses a method for producing a thick high-strength steel plate having a coarse-grained ferrite layer on each of the upper and lower surfaces, the method including performing rolling at a cumulative rolling reduction of 2% or more and a temperature of Ac1−50° C. or lower in a cooling step after hot rolling a cast slab containing, on a mass percent, 0.01%-0.3% C, 0.6% or less Si, 0.2%-2.0% Mn, 0.06% or less Al, 0.005%-0.035% Ti, and 0.001%-0.006% N; heating the steel sheet to a temperature exceeding Ac1 and less than Ac3; and allowing the steel sheet to cool. The technique described in JP '936 seems to contribute to improvement in the SCC sensitivity, weather resistance, and corrosion resistance of a steel material, and to the suppression of the degradation of the material after cold forming. However, in the technique described in JP '936, disadvantageously, the reheating step is needed, making the production process complex. Furthermore, it is necessary to install a reheating apparatus and so forth.
In recent years, steel pipes to be used in a very cold land have often been required to have excellent fracture toughness, in particular, crack tip opening displacement characteristics (CTOD characteristics) and drop weight tear test characteristics (DWTT characteristics), from the viewpoint of preventing the burst of a pipeline.
For such a request, for example, Japanese Unexamined Patent Application Publication No. 2001-207220 discloses a method for producing a hot rolled steel sheet for a high-strength electric resistance welded steel pipe, the method including heating a steel slab containing, on a mass percent, C, Si, Mn, and N in an appropriate amount, Si and Mn in such a manner that Mn/Si satisfies 5 to 8, and 0.01%-0.1% Nb; performing rough rolling under conditions in which the reduction rate of first rolling at 1100° C. or higher is 15% to 30%, the total reduction rate at 1000° C. or higher is 60% or more, and the reduction rate of final rolling is 15% to 30%; cooling the steel sheet at a cooling rate of 5° C./s or more in such a manner that the temperature of a surface layer portion reaches the Ar1 point or lower; initiating finish rolling when the temperature of the surface layer portion reaches (Ac3−40° C.) to (Ac3+40° C.) by recuperation or forced heating; terminating the finish rolling under conditions in which the total reduction rate is 60% or more at 950° C. or lower and in which the rolling end temperature is the Ara point or higher; initiating cooling after 2 seconds of the termination of the finish rolling to cool the steel sheet to 600° C. or lower at a rate of 10° C./s or more; and coiling the steel sheet at 600° C. to 350° C. A steel sheet produced by the technique described in JP '220 seems to be formed into a high-strength electric resistance welded steel pipe having a fine microstructure of a surface layer of the steel sheet and excellent low-temperature toughness, in particular, excellent DWTT characteristics, without adding an expensive alloy element or performing heat treatment of the entire steel pipe. However, in the technique described in JP '220, in the case of a steel sheet having a large thickness, a desired cooling time is not ensured. To ensure desired properties, further improvement in cooling capacity is disadvantageously needed.
Japanese Unexamined Patent Application Publication No. 2004-315957 discloses method for producing a hot rolled steel strip for high-strength electric resistance welded steel pipe having excellent low-temperature toughness and excellent weldability, the method including heating a steel slab containing, on a mass percent, C, Si, Mn, Al, and N in appropriate amounts, 0.001%-0.1% Nb, 0.001%-0.1% V, and 0.001%-0.1% Ti, and one or two or more of Cu, Ni, and Mo, the steel slab having a Pcm value of 0.17 or less; terminating finish rolling under conditions in which the surface temperature is (Ar3−50° C.) or higher; thereafter rapidly cooling the steel sheet; coiling the steel sheet at 700° C. or lower; and performing slow cooling.
However, in recent years, a steel sheet for a high-strength electric resistance welded steel pipe has been required to have further improved low-temperature toughness, in particular, the CTOD characteristics and the DWTT characteristics. In the technique described in JP '957, the low-temperature toughness is not sufficient. That is, unfortunately, the resulting steel sheet does not have excellent low-temperature toughness enough to satisfy CTOD characteristics and DWTT characteristics required.
Disadvantageously, a hot rolled steel sheet in the related art varies widely in material properties at points in the longitudinal direction and width direction of the sheet, in many cases.
It could therefore be helpful to provide a thick-walled high-strength hot rolled steel sheet for high strength electric resistance welded steel pipe or a high strength spiral steel pipe, the steel sheet having a high tensile strength TS of 510 MPa or more and excellent low-temperature toughness, in particular, excellent CTOD characteristics and DWTT characteristics, and to a method for producing the steel sheet without the need for the addition of large amounts of alloy elements.
It could also be helpful to further improve the uniformity of a material in the longitudinal direction and the width direction of the sheet.
It could further be helpful to provide a thick-walled high-strength hot rolled steel sheet having excellent uniformity of the material and an appropriate surface microstructure without a local increase in strength or the deterioration in ductility or toughness.
It could still further be helpful to provide a thick-walled high-strength hot rolled steel sheet having an appropriate surface microstructure and excellent uniformity of the microstructure in the thickness direction.
The term “excellent CTOD characteristics” used here indicates that a critical opening displacement (CTOD value) is 0.30 mm or more when a CTOD test is performed at a test temperature of −10° C. in conformity with the regulation of ASTM E 1290. The term “excellent DWTT characteristics” used here indicates that in the case where a DWTT test is performed in conformity with the regulation of ASTM E 436, the lowest temperature (DWTT temperature) when the percent shear fracture is 85% is −35° C. or lower.